The present invention is related generally to industrial process control transmitters, and particularly to a sensor excitation circuit for use in such transmitters.
Industrial process control transmitters are used to measure process variables in field locations and provide standardized transmission signals as a function of the measured variable. The term xe2x80x9cprocess variablexe2x80x9d refers to a physical or chemical state of matter or conversion of energy, such as pressure, temperature, flow, conductivity, pH, and other properties. Process control transmitters are often operated in hazardous field environments to measure these variables and are connected by two-wire communication lines to a central or control station.
One such transmitter is described in U.S. application Ser. No. 09/312,411 filed May 14, 1999, now U.S. Pat. No. 6,295,875 granted Oct. 2, 2001, by Roger L. Frick and David A. Broden for xe2x80x9cPressure Sensor for a Pressure Transmitterxe2x80x9d, and assigned to the same assignee as the present invention. The Frick et al. transmitter employs a capacitive sensor having a deflectable sensing diaphragm and three or more capacitor electrodes forming separate capacitors with the diaphragm. Two of the capacitors are primary sensing capacitors that are arranged differentially so that the capacitances of the primary sensing capacitors change oppositely in proportion to the process variable. The third (and fourth, if used) capacitor is a compensation capacitor that provides signals representing certain offset errors, or hysteresis, associated with one or both of the primary capacitors.
The Frick et al. transmitter includes a sigma-delta converter that acts as a capacitance-to-digital converter. An excitation circuit provides a charge packet to the capacitors of the sensor, which are charged by an amount based on the capacitance value of the capacitor. The charge is transferred to an integrator/amplifier of the sigma-delta converter to derive a signal representative of sensor capacitance. The signal is processed and a standardized transmission signal is transmitted to the central control station via the two-wire communication lines.
The excitation circuit of the Frick et al. application includes separate, external operational amplifiers that invert the representations of the charges on the compensation capacitors and apply a gain adjustment to the charge. The separate operational amplifiers in the excitation circuit limit sampling frequency, introduce noise and consume precious power. Additionally, current operational amplifiers require substantial portions of the sample cycle for the output voltage to settle (slew). The consumption of current during this long settling time, results in insufficient current being available for other purpose, such as for diagnostics.
The present invention is directed to an industrial process control transmitter that employs integrated inverting amplifiers that require less current to settle the output signal. Consequently, more current is available for diagnostic and other purposes. Additionally, an auto-zeroing switch reconfigures the integrated inverting amplifiers to unity gain amplifiers so that less current is required for settling the output voltage and more current is available for diagnostic and other purposes.
The industrial process control transmitter has a capacitive sensor adapted to monitor a process condition. The sensor has at least two primary sensing capacitors that respond oppositely to the process condition, and at least a third compensation capacitor that responds to error or hysteresis in a manner differently from either primary sensing capacitor. A sensing circuit includes a charge summing node, an integrated inverting charge amplifier coupling the compensation capacitor to the summing node, and a sigma-delta capacitance-to-digital converter circuit coupled to the summing node providing a digital output representative of the process condition. A transmitter output circuit receives the digital output and generates a standardized transmitter output adapted for coupling to a remote receiver.
The capacitors are charged by a charge circuit that operates in phases such that a sensing capacitor is charged during one phase and its charge is transferred to the sigma-delta circuit during a second phase. In preferred embodiments the second phase has a longer time duration than the first phase, so the output of the inverting amplifier has more time to settle and the amplifier requires less current than prior amplifiers.
In another embodiment, the integrated inverting amplifier is shared by two compensation capacitors operated during different sensing cycles.